I looked up MW but failed to see whether they are interchangeable.
On a daily basis I find difficulties in adjective forming, so I guess this is just one more of the -IC versus -AL dilemmas, no?
Numeral (noun) is the shape that draw on the paper to represent a number.
Numeric (adj) lets us know that some noun represents some number.
0 and O look almost identical in many fonts. But they are not the same. The letter form O has an orthographic (spelling) value, but no numeric value, although it can be used to spell many great words like book and hog.
Contrast that with the numeral 0. You can't spell anything with it, but it does have a numeric value, approximately halfway between -1 and 1.
In some fonts, you can tell the two apart:
0 is the numeral, O is the letter form with no numeric value.
In Rome, on the other hand, the letter form V also served as a numeral, so you could either use it to spell things, or to represent numeric values.
...and one more take on this, from the comments:
Forty-seven is a number, expressed without the use of numerals. 47 is the same number, expressed using numerals.
Boobs is a word referring to mammalian anatomy.
80085 is the punchline to a adolescent calculator joke that uses numerals to suggest the word "boobs." The numeric value of 80085 is not important to the joke.
Numeric is often used in something like this: "0 has no numeric value", but you would use Numeral in a sentence like this "II is two in roman numerals".
Numeric - Indicating a number or value.
Numeral - A word number or symbol expressing a number